The present invention relates to an image display apparatus and, more particularly, to a head- or face-mounted image display apparatus that can be retained on the observer's head or face.
For a head-mounted image display apparatus, it is important to minimize the overall size and weight thereof in order to make the observer feel comfortable when wearing it. An essential factor in determining the overall size of the apparatus is the layout of the optical system.
FIG. 15 shows the optical system of one conventional head-mounted image display apparatus Japanese Patent Application Laid-Open (KOKAI) No. 3-101709 (1991)!. In this image display apparatus, an image that is displayed by a two-dimensional image display device is transmitted as an aerial image by a relay optical system including a positive lens, and the aerial image is projected into an observer's eyeball as an enlarged image by an ocular optical system formed from a concave reflecting mirror. There has also been known a direct-vision layout in which an enlarged image of a two-dimensional image display device is observed directly through a convex lens. With these conventional layouts, the amount to which the apparatus projects from the observer's face is unfavorably large. Further, it is necessary in order to provide a wide field angle for observation to use a positive lens having a large diameter and a large two-dimensional image display device. Accordingly, the apparatus inevitably becomes increasingly large in size and heavy in weight.
To enable the observer to use the image display apparatus for a long time without fatigue and to attach and detach the apparatus with ease, it is preferable to adopt an arrangement in which a short and lightweight ocular optical system is disposed immediately in front of the observer's eyeball. With this arrangement, a two-dimensional image display device, an illuminating optical system, etc. can be disposed so that the apparatus projects forwardly from the observer's head to a minimal extent. Thus, it is possible to reduce the amount of projection of the apparatus and also the weight thereof.
Next, it is necessary to ensure a wide field angle in order to enhance the feeling of being at the actual spot which is given to the observer when viewing the displayed image. In particular, the stereoscopic effect of the image presented is determined by the angle at which the image is presented see The Journal of the Institute of Television Engineers of Japan Vol. 45, No. 12, pp. 1589-1596 (1991)!. The next matter of great concern is how to realize an optical system which provides a wide field angle and high resolution. It is known that it is necessary in order to present a stereoscopic and powerful image to the observer to ensure a field angle of 40.degree. (.+-.20.degree.) or more in the horizontal direction, and that the stereoscopic and other effects are saturated in the vicinity of 120.degree. (.+-.60.degree.). In other words, it is preferable to select a field angle which is not smaller than 40.degree. and which is as close to 120.degree. as possible. However, in a case where the above-described ocular optical system is a plane reflecting mirror, it is necessary to use an extraordinarily large two-dimensional image display device in order to make light rays incident on the observer's eyeball at a field angle of 40.degree. or more. After all, the apparatus increases in both the overall size and weight.
Further, since a concave mirror produces a strong concave curvature of field along the surface of the concave mirror because of its nature, if a planar two-dimensional image display device is disposed at the focal point of a concave mirror, the resulting observation image surface is curved, so that it is impossible to obtain an image for observation which is clear as far as the edges of the visual field.
Thus, it has heretofore been difficult to realize a compact and lightweight image display apparatus which is capable of presenting the observer an image which is clear as far as the edges of the visual field at a wide field angle.
In view of the above-described problems of the conventional technique, the present invention aims at providing a compact and lightweight image display apparatus having high resolution and a large exit pupil diameter, which enables observation of an image at a wide field angle.
The problems associated with the conventional technique will be explained below more specifically. Aberrations which are produced in an ocular optical system formed from a decentered concave reflecting mirror may be roughly divided into those which directly affect the image-forming performance, e.g., field curvature, astigmatism, coma, etc., and pupil aberration, which is not directly related to the image-forming performance.
The aberrations which directly affect the image-forming performance can be corrected to a certain extent by employing a decentered surface or an aspherical surface in the relay optical system. However, pupil aberration, which is not directly related to the image-forming performance, cannot be corrected by using a decentered surface or an aspherical surface in the relay optical system. This means that the entrance pupil of the ocular optical system, which is in imagery relation to the observer's pupil position, fails to coincide with the exit pupil of the relay optical system, and that the exit pupil diameter of the relay optical system must be enlarged to a considerable extent. If the pupil diameter of the relay optical system is not satisfactorily large, the edges of visual field is eclipsed, and no adequate resolution can be obtained. To cope with these problems, the pupil diameter of the relay optical system must be enlarged. However, an increase in the pupil diameter of the relay optical system causes the load on the relay optical system to be considerably increased. As a result, the relay optical system becomes disadvantageously large in size and complicated in arrangement.
The way in which pupil aberration occurs will be explained below with reference to FIG. 12, in which illustration of a relay optical system is omitted. FIG. 12 shows pupil aberration produced by a conventional ocular concave mirror, which is a surface-coated mirror. In the figure, reference numeral 1 denotes an observer's pupil position, 2 a concave reflecting mirror, 3 an observer's visual axis lying when the observer sees forward, and 4 a pupil position where an image of the observer's pupil is projected by the concave mirror 2. A coordinate system is defined with respect to the drawing in such a way that the plane of the drawing is taken as YZ-plane, and a perpendicular extending from the obverse surface to the reverse surface of the drawing in a direction perpendicular to the plane of the drawing is taken as X-axis. The figure shows ray tracing carried out with the observer's pupil diameter assumed to be 12 millimeters.
FIG. 13(a) shows ray tracing in which only axial rays are traced with the observer's pupil taken as an object point. In FIG. 13(a), reference numeral 1 denotes an observer's pupil position, 2 an ocular concave mirror, 3 an observer's visual axis lying when the observer sees forward, and 4 a pupil position where an image of the observer's pupil is projected by the concave mirror 2, in the same way as in the case of FIG. 12. FIG. 13(b) shows where chief rays 11 to 15 in FIG. 13(a) lie in the image field according to the field angle.
FIG. 14(a) shows the rays in FIG. 13(a) as projected on the XZ-section. In FIG. 14(a), reference numeral 1 denotes an observer's pupil position, 2 an ocular concave mirror, and 3 an observer's visual axis lying when the observer sees forward, in the same way as in the case of FIGS. 12 and 13(a). FIG. 14(b) shows where rays 21 to 23 and 31 to 33 in FIG. 14(a) lie in the image field according to the field angle.
It will be understood from FIG. 14(a) that the ray 21 in particular has aberration in the Z-axis direction. Thus, it will be understood that in FIG. 12 a relay lens having a large effective aperture and satisfactorily corrected for aberration is needed in order to enable an image lying leftward of the observer's right eye to be displayed without being eclipsed, and hence the load on the relay lens increases.